Leafr: Lookup Expressions And Find Rewrites
Leafr is a language based on pattern matching and string rewriting. Created for Langjam 2021 under the theme "first-class comments", the language aims to treat comments as first class by allowing data to be provided in the form of written prose.
It's inspired by Refal.
How to Run
Leafr is written in Ruby 2.6.3.
Usage: ruby leafr.rb [options]
-r, --run FILE The path of a file to run
-d, --debug Enable debug printing
-p, --play Play a terminal animation of the code running
-t, --tests Run the test suite used to develop the language
-h, --help Display this help
First class comments
When we considered what "first class comments" could mean, they idea came to us that this would mean comments are a core part of the logic of a program. What if you could write comments in your code, and have a series of rules fill in the logic behind them? Here is an example of that in Leafr
function my_function(str) {
// call the reverse method on "str" and print that out
// do all of that 10 times
}
-------------
// call the <X> method on "<Y>"
<Y>.<X>()
<> <X> and print that out
<> console.log(<X>)
{<A>// do all of that <X> times<Z>}
{
for (let i = 0; i < <X>; i++) {
<A>
}
<Z>
}
yields
function my_function(str) {
for (let i = 0; i < 10; i++) {
console.log(str.reverse())
}
}
Syntax
The syntax of Leafr is very simple. All files are broken into two sections, the "data" and the "code". These sections are separated by at least three - characters, like this.
Here is the data
---
Here are the
rules
Rules are the only control construct in Leafr. A rule is a simple find and replace. Every line that begins with no spaces is part of a "rule header", and defines the pattern that will be matched. Following lines indended by two spaces define the rewrite. For example:
Hello
World
--------
Hello
Goodbye
will yield
Goodbye
World
The syntax <A> defines a pattern. Patterns can be used in both headers and rewrites. Any patterns matched in the header can be used in the rewrite. So
Hello
World
-----
Hello
<X>
Goodbye
<X>
is equivalent to the above.
If a pattern is a lower case letter, it will match only one character. Otherwise it matches N or more charaters, including whitespace.
The pattern <> is removed before a rule is run. IT can be used in cases where a rule would otherwise parse incorrectly, or as a placeholder to make whitespace obvious.
Leafr is a turing complete language, as shown by the gif above implementing Rule 110.
Matching rules
Rewrites are chosen based on the following rule order
- Shortest rewrite
- Fewest variable substitutions
- Defined order
Samtara Team Members
